1. Field of the Invention
The present invention relates to a cathode material comprising a complex oxide including lithium (Li), manganese (Mn), chromium (Cr) and at least one kind selected from the group consisting of titanium (Ti), magnesium (Mg) and aluminum (Al), a method of manufacturing the cathode material, and a battery using the cathode material.
2. Description of the Related Art
In recent years, with significant advances of various electronic devices, studies of rechargeable secondary batteries as power sources which can be conveniently and economically used for a long time have been progressing. As typical secondary batteries, lead-acid batteries, alkaline batteries, lithium secondary batteries and the like are known. Among them, the lithium secondary batteries have advantages that higher power and higher energy density can be achieved.
The lithium secondary batteries comprise a cathode capable of reversibly inserting and extracting lithium ions, an anode and an electrolyte. As a cathode material, for example, a metal oxide, a metal sulfide or a polymer is used. More specifically, a compound not including lithium such as TiS2, MoS2, NbSe2 or V2O5, a lithium complex oxide including lithium such as LiMO2 (M=Co, Ni, Mn, Fe or the like) or LiMn2O4 is known.
Among them, LiCoO2 is widely and practically used as a cathode material having a potential of approximately 4 V relative to a lithium metal potential, and is an ideal cathode material in various aspects because LiCoO2 has a higher energy density and a higher voltage. However, Co (cobalt) as a resource is unevenly distributed and scarce, so there is a problem that it is difficult to stably supply Co, thereby a material cost becomes higher.
Therefore, instead of LiCoO2, a cathode material including abundant and low-cost nickel (Ni) or manganese (Mn) as a base holds promise.
Although LiNiO2 has a large theoretical capacity and a high discharge potential, its crystalline structure collapses in accordance with the progress of a charge-discharge cycle, thereby resulting in a decline in a discharge capacity and lower thermostability.
Moreover, LiMn2O4 with a normal spinel structure has as high a potential as LiCoO2, and can obtain a high battery capacity. Further, LiMn2O4 can be easily synthesized. However, there are problems such as insufficient stability and insufficient cycle characteristics, that is, degradation in capacity is large during storage at high temperature, and manganese is dissolved in an electrolyte solution.
Moreover, although LiMnO2 with a layer structure can obtain a higher capacity than LiMn2O4, there are problems that it is difficult to synthesize LiMnO2, and when a charge-discharge cycle is repeated, the structure becomes unstable, and the capacity declines.
In order to overcome the problems, Li2+xMn0.91Cr1.09O4 in which a part of manganese is substituted with chromium and a large amount of lithium is included has been proposed as a cathode material (refer to, for example, Electrochemical and Solid-State Letters Vol. 3, No. 8, p. 355, 2000). However, in the cathode material, there is a problem that cycle characteristics at room temperature are not sufficient. Moreover, the cathode material is formed through synthesizing Mn0.91Cr1.09O4 by reaction in a solution using methanol as a dispersion medium, and then firing a lithium salt and Mn0.91Cr1.09O4 in an atmosphere of nitrogen, so the cathode material has disadvantages that toxic methanol is used, and two-step reaction is required.
Moreover, in Journal of The Electrochemical Society Vol. 149, No. 4, p. A431, 2002, Li1.2Mn0.4Cr0.4O4 is proposed as a cathode material. In the cathode material, when charge is performed until reaching 4.4 V at a voltage relative to lithium, and then discharge is performed until reaching 2.5 V, an initial charge capacity of 258 mAh/g, and an initial discharge capacity of 173 mAh/g can be obtained; however, cycle characteristics at room temperature are not sufficient.